["Journal of Child Psychology and Psychiatry, EarlyView. ", "\n\nBackground\nSelf‐harm is a pressing public health issue, particularly among adolescents. Physical activity is associated with reduced self‐harm among youths, but evidence is limited. Most studies rely on self‐reported measures and are primarily focused on moderate‐to‐vigorous physical activity (MVPA), often overlooking the role of light‐intensity physical activity (LIPA). To address these gaps, we investigated the prospective associations between accelerometer‐measured 24‐hr movements and subsequent self‐harm in adolescents.\n\n\nMethods\nThis prospective cohort investigation analysed 2,016 children from the UK Millennium Cohort Study using data from the age 7 years survey (MCS4) when accelerometers were introduced (mean age = 7.23 years, SD = 0.25). Accelerometer‐measured duration of MVPA, LIPA, sedentary behaviour (SB), and sleep were analysed. Self‐harm behaviours were assessed via validated self‐report questionnaires at ages 14 and 17 years, and subsequently, the associations examined using logistic and compositional data models.\n\n\nResults\nAmong 2,016 participants (48.6% female), 548 self‐harm cases occurred during 18,696 person‐years of follow‐up, with a median follow‐up duration of 8.5 years (cumulative incidence = 27.2%). LIPA was inversely associated with self‐harm (OR 0.76, 95% CI: 0.65–0.88). This association was stronger in compositional models (OR 0.23, 95% CI: 0.11–0.47). MVPA showed a significant non‐linear, inverted U‐shaped association with self‐harm (p for non‐linearity = .02). The risk peaked at approximately 1.2 hr/day and decreased substantially at levels exceeding 1.7 hr/day, while SB and sleep were not significantly associated with self‐harm. Stratified analyses confirmed robustness.\n\n\nConclusions\nOur findings reveal a pronounced inverse association between LIPA and self‐harm, along with a non‐linear association for MVPA. These results support 24‐hr movement guidelines that emphasise activity transitions over isolated targets and highlight LIPA's role in early prevention strategies.\n\n"]